I'm a developer/system admin/unix guy so I have many IDEs/Editors/Terminals running and I'm also temperature freak... I like it cold, here are a few tips I did to keep my MacBook Pro Unibody under 46 degrees C ~ 115 degrees F. Please list your tips.

Clean your MacBook Pro fans from within... it's simple, open the bottom closure and blow the hell out of these fans.

Download and install Fan Control and run your fans @ 2800-3000 RPM

Position your MacBook on a hard surface

Ditch all browsers to Safari, i tried them all Firefox up to version 5 and 7 (Aurora), Chrome and Opera... nothing beats the CPU usage (less of it) than Safari. Also flash youtube videos run with less resources on Safari for some reason. Block unneeded content (Ads)

If possible - don't use Adobe Flash

Use your faster GPU on AC Power, and lower on Battery power (use gfxCardStatus for source-based GPU switching)

Wouldn't running your screen at less than 90% help more, if at all? Also, my (pre-Unibody) MBP usually runs at ~170 degrees F, with no issues at all. I don't think heat is as big an issue as some make it out to be.
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CajunLukeJul 13 '11 at 17:41

Safari FTW, it also uses half as much memory as the next nearest browser (I've tested Chrome, FF3, and FF4)
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OdinulfJul 13 '11 at 18:13

Many SSD use more idle power than HDD so I would disagree that replacing that component is a sure fire way to reduce heat generation. (especially for people that have use cases where the HDD can spin down)
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bmike♦Jul 13 '11 at 18:14

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Haytham - why don't you take each of your answers and edit / move them to official answers. They you can see those for which people vote most.
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bmike♦Jul 13 '11 at 18:30

bmike, the heat doesn't come from the power consumption as much as by the rotating disk motor. I guess the SSD tip is subjective... more tests need to be made. Concerning your suggestion, virtually anything done to cool down your system might be worth doing... so no voting required.
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Haytham ElkhojaJul 13 '11 at 19:48

7 Answers
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I tend to remain sceptical of anyone making claims without valid and reliable data. With that said, some practices you list are of critical importance; others, I'm not sold on one bit. Please don't take this a slight against you, but rather as a criticism of your general claims (and lack of scientific data therein). Moreover, if you enacted all those changes at once, you'll never know which one made a real difference, and which one did absolutely nothing. Proper scientific methodology calls for the manipulation of one variable against a control (unless you want to run the more complex ANOVA).

Clean your MacBook Pro fans from within... it's simple, open the
bottom closure and blow the hell out of these fans

Absolutely. This is by far the most effective way to ensure your system continues to operate nominally over the years. It is sadly also the most neglected. Dust sticks to the blades (and most of the internals) creating a "sticky" residue and not only prevents the fan's blades from spinning at their optimum, but seriously restricts air flow. Computers generally accumulate far more dust than people expect. Regular cleanings are essential!

Download and install Fan Control and run your fans @ 2800-3000 RPM

This of courses comes at a sacrifice to noise (increasing it) and I'd like to see data on how much cooler the chips operate with such a small increase to RPM.

Ditch all browsers to Safari, I tried them all Firefox up to version 5
and 7 (Aurora), Chrome and Opera... nothing beats the CPU usage (less
of it) than Safari. Also flash youtube videos run with less resources
on Safari for some reason

Again, I'd like to see some data being this claim. These guys have conducted some very loose tests that support your assertion, and I'm inclined to agree that Safari, being so tightly integrated into OS X should net improvements over its competitors across the board, but a few CPU cycles here and there aren't going to amount to a drop (drastic or modest) in temperatures (these saw extremely small differences in performance).

GPUs put off heat, wether they're integrated or discrete. I can see a reduction in power, but a reduction in heat, again, I'm not sold. In the case of MBP, models that possess discrete video cards have dual fans (one for the CPU and one for the GPU). So using the discrete card may kick off additional heat, but it's also got a dedicated fan to push that heat away from the machine. Stressing the integrated chip will see the CPU spike in temperatures, which means that fan will be doing more work to handle the additional load. It may be the case that the fan on the CPU (together with the integrated GPU) may be sufficient in pulling away the excess heat, but at no doubt the cost of excessive noise (generated by 6,000 RPMs). I don't know if anyone has done testing, but from my own anecdotal experience with my old MBP1,1, the GPU fans rarely switched gears (unless playing intensive games and then the issue would be moot as you'd want to use the discrete video). It was always the CPUs fans that took the brunt of the work.

Replace your internal hard drive with an SSD

Not only will this greatly improve performance, but it can't hurt in reducing temperatures. The idle/active power consumption of an SSD is about 0.5W to 2w, respectively. The average power consumption of a traditional HDD is about double that (1W to 2.5W, respectively). I won't know what that transfers to in the temperatures department, but it certainly can't hurt.

I'd love to see data on this. LED backlighting is already quite efficient and distributing heat, but I'm not sure a reduction (at least that small) in power would have any effect. As for the keyboard lights, those are LEDs and as is with such small LED bulbs, they are incredibly power efficient and produce little to no heat as a result.

With all that said, I'd saying re-application of Arctic Silver 5 (or thermal compound of equal quality) and the cleaning of dust from the internals is the most effective (and proven) way to keep your system cool. I've never seen data to convince me that turning off some service, or placing your system on a pad with a couple of fans does anything (other than perhaps keep the case cooler to the touch) worth merit.

The issue isn't with flash per se, i have many tabs that remain open (and idle) throughout the day (or the week), mostly Gmail, FB, Google Reader and now Google+ sigh, and from my tests, nothing beats Safari when it comes to cpu usage while idling.
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Haytham ElkhojaJul 13 '11 at 19:51

I agree with safari being good on low CPU usage at idle - but I feel that uninstalling flash will make your mac run cooler than if you leave it installed.
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bmike♦Jul 13 '11 at 22:36